Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Mon May 06 2024
Valid 12Z Fri May 10 2024 - 12Z Tue May 14 2024
...Wet/snowy pattern along the southern coast and Panhandle with
heaviest precipitation late this week through the weekend...
...Overview...
Today's guidance continues to show fairly persistent mean
troughing aloft aligned over or near the western mainland while
the most prominent embedded upper low should be between the
southeastern Bering Sea and Gulf of Alaska. This pattern will
likely support significant precipitation along the southern coast
and Panhandle late week into the weekend with lighter activity
lingering into next week. Behind a Gulf of Alaska wave expected
to bring a burst of heavy rain and some high elevation snow to the
Panhandle and Southcentral coast on Thursday (just before the
start of the extended period), the primary system of note should
track from the eastern Aleutians into the Gulf of Alaska Friday
onward, focusing heaviest rainfall and some mountain snow from the
Alaska Peninsula to the Kenai Peninsula while lesser amounts
ultimately reach as far east as the Panhandle. By next Tuesday
the guidance develops mixed signals regarding a potential system
that could affect the Aleutians and surrounding waters.
...Guidance Evaluation and Preferences...
For the system currently forecast to track from the eastern
Aleutians into the Gulf of Alaska Friday onward, model/ensemble
trends over the past couple days have generally led to improved
clustering toward earlier ECMWF/ECens guidance that has been on
the northern side of the envelope. While the 12Z UKMET adjusted
to conform to the ECMWF cluster fairly well, recent GFS runs have
been showing alternative details in one way or another. The 12Z
run placed more emphasis on residual Bering Sea energy, leading to
a slower eastward drift of the surface/upper low than the
dynamical consensus and latest ECMWF-initialized machine learning
(ML) models. 06Z and new 18Z GFS runs quickly bring upstream
Pacific dynamics into the area, generating a second vigorous
surface low. Thus far other dynamical guidance and the MLs do not
offer much support for the possibilities offered by GFS runs.
Consensus says that the emerging western Pacific dynamics may
support weak/elongated surface low pressure to the south of the
western Aleutians early in the period and then track over the
mid-latitude Pacific.
This trailing Pacific energy does pose some uncertainty for the
Northeast Pacific forecast late in the period. At the time of
forecast preparation, the 12Z CMC was on its own for the strength
and northern track of associated low pressure. In later guidance,
the 18Z GFS eventually brings a wave to a position just south of
the Gulf of Alaska by late next Tuesday. The ECMWF and new 12Z
MLs offer a more muted signal for some waviness that could at
least have some influence on moisture reaching the Panhandle.
Very low predictability with this aspect of the forecast favors a
conservative approach for the time being.
By day 8 next Tuesday the guidance diverges for upper level and
surface details over the Aleutians/Bering Sea. The ECMWF/CMC are
most pronounced with combined definition and eastward progression
of an upper shortwave/surface system. Latest GFS runs are slower
(though trending gradually faster), while the ensemble means and 4
of 5 MLs hold onto stronger upper ridging and higher surface
pressures over the region. An even mix of the means and 12Z
ECMWF/GFS yields a warm front that offers a reasonable
extrapolation of its upstream progress through day 7.
Aforementioned guidance comparisons led to starting the first half
of the forecast with 40 percent of the 12Z ECMWF and 20 percent
each of the GFS/UKMET/CMC. With the UKMET dropping out after
ending its run early Sunday and confidence lacking in the CMC's
Pacific details, days 7-8 Monday-Tuesday transitioned to 40-50
percent ensemble mean input (12Z GEFS/00z ECens). Relative weight
of the GFS/ECMWF also evened out by day 8 to provide the desired
result over the Aleutians/Bering Sea.
...Weather/Hazards Highlights...
Immediately before the start of the extended period, guidance
continues show a brief period of heavy precipitation over the
Panhandle and Southcentral coast on Thursday with a wave tracking
quickly northward into the Gulf of Alaska and on the
north/northeast. While latest model runs have nudged the surface
track a little east of prior consensus, continuity remains good
with the signal for anomalously high precipitable water values to
focus over the northern Panhandle in particular. Corresponding to
this threat which should be greatest over coastal/lower elevation
areas, the Days 3-7 Hazards Outlook depicts a heavy rain area from
the Panhandle back through the Southcentral coast. Some mountain
snow is likely as well. The next storm of interest should track
eastward from the eastern Aleutians from Friday onward. Brisk
southeasterly low level flow ahead of this storm should help to
enhance precipitation focus along coastal/windward terrain along
the Alaska Peninsula, Kodiak Island, and Kenai Peninsula during
Friday-Saturday. The Hazards Outlook depicts a heavy rain area
for this event as well. Expect precipitation with lesser totals
to extend eastward through the Panhandle during the weekend. On
the back side of the surface low, the Aleutians and Alaska
Peninsula may see a period of brisk northwesterly winds. The
forecast pattern should favor some precipitation to linger along
the southern coast and Panhandle into the first half of next week.
Currently the most likely scenario would keep amounts on the
light to moderate side, but there is a lower probability that a
wave reaching the Northeast Pacific early next week or soon
thereafter could raise amounts some. The remainder of the
mainland will tend to see a showery pattern given the anticipated
upper trough axis to be aligned over western areas. Details over
the Aleutians and vicinity become more uncertain by early next
week, with potential for a warm front or better defined system to
bring some rain into the region.
The unsettled pattern will tend to keep diurnal temperature ranges
more narrow than climatology, with daytime highs most likely to be
below normal and morning lows averaging above normal. Some
pockets of modestly above normal highs may be possible over the
Interior, while various locations depending on the day may see
slightly below normal lows.
Rausch
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html